Upwelling and Downwelling Driven by the North Equatorial Countercurrent and Internal Waves at Hatohobei Island and Helen Reef, Palau

Upwelling and downwelling at two near-equatorial islands in the western Pacific, Hatohobei Island and Helen Reef, Palau, are investigated using time series from a temperature array in the upper 57 m from 2014 to 2019 and a ship survey from 3 to 5 July 2013. During the survey, on the upstream side, upwelling/downwelling is found above/below an eastward subsurface current maximum due to an unusual mesoscale eddy. This eddy detached from a meander in the eastward North Equatorial Countercurrent in May 2013 and translated southwestward to the islands over 2 months bringing among the strongest currents seen at these islands in altimetry from 1993 to 2020. Following a streamline, the flow satisfied the Bernoulli equation outside of the wake. In both the cyclonic and anticyclonic wakes from the south and north points of Helen Reef, elevated estimated diffusivities from a shear-based parameterization were similar at 10(-5) m(2) s(-1) and a factor of 10 above background values. In the time series, downwelling in the upper 57 m is associated with a broader, surface-intensified NECC converging at topography. Also, the shallower thermocline in 2014 and during the El Ninos of 2015-2016 and 2018-2019 led to increased stratification at this depth and permitted greater internal wave activity. Peaked semidiurnal internal waves with peak-to-peak amplitudes of 4 degrees C were equal to interannual changes. The net downwelling/warming of 0.5 degrees C at the temperature array when smoothed over 2 months is consistent with both current-driven downwelling and internal-wave driven mixing of warmer waters downwards.